Various pneumatic tires have been proposed which contain a built-in sealant layer based upon a depolymerized butyl rubber layer. For example, see U.S. Pat. Nos. 4,895,610, 4,228,839, 4,171,237, 4,140,167 and U.S. patent application Ser. Nos. 10/171,057, 10/368,259 and 2005/0205186.
Additional patent publications which propose various tire constructions which may involve built-in or built-on sealants for tires such as for example, U.S. Pat. Nos. 1,239,291, 2,877,819, 3,048,509, 3,563,294, 4,206,796, 4,286,643, 4,359,078, 4,444,294, 4,895,610, 4,919,183 and 4,966,213.
In practice, as the tire is run, centrifugal force can promote a small degree of flow of the built-in sealant layer located in the shoulder or sidewall regions of the tire toward the center, or crown region, of the tire, thereby reducing the puncture sealing capability of the built-in sealant layer in the shoulder region of the tire.
For this invention, it is proposed to provide a tire having a unitary built-in sealant layer divided into zones, namely an annular central zone positioned in the crown region of the tire and annular lateral zones, wherein the built-in sealant of the lateral zones are of a higher storage modulus (G′) than the sealant of the central zone to thereby present a greater dimensional stability and thereby resistance to flow of the sealant in the lateral zones promoted by centrifugal force resulting by movement of the associated tire.
In one embodiment, the lateral zones of the sealant layer may be individually positioned on each axial side of the central zone.
In another embodiment, the lateral zones of the sealant layer may be individually positioned axially outward from the center of the sealant layer and on the central sealant zone and wherein the said central zone extends over the entire axial width of the sealant layer.
In practice, it is desirable for the storage modulus (G′), at a 5 percent dynamic strain at 100° C. and 1 hertz of the sealant composition of the lateral sealant zones to be least 15, alternately at least 20, kPa higher (greater) than the sealant composition of said central sealant zone.
It is to be appreciated, that in one embodiment, the zones of the built-in sealant layer for the pneumatic tires, which is derived from a depolymerization of a butyl rubber-based sealant precursor composition typically, contain a rubber reinforcing carbon black filler to render the sealant black in color or may contain precipitated silica with only a minimal amount of carbon black, if any, preferably exclusive of carbon black, together with a colorant to color the sealant layer a color other than black.
For such zoned sealant layer, organoperoxide efficiency for the in situ depolymerization of the butyl rubber is of interest.
In one aspect, by controlling the use of the organoperoxide, free radical promoted butyl rubber depolymerization activity, or rate, (referred to herein as organoperoxide activity) may be varied and the degree (extent) of depolymerization of the butyl rubber varied, depending upon the selection of organoperoxide for the individual sealant zones to thereby result in the sealant compositions of the individual sealant zones having different storage modulus (G′) values.
Accordingly, on this basis, an organoperoxide for the sealant precursor of the central zone is to have a greater activity (a more active organoperoxide) than the organoperoxide of said lateral zones.
For example, where the organoperoxide for the lateral zones is comprised of dicumyl peroxide (a herein preferred organoperoxide for such purpose) for the in situ formation of the built-in sealant of the lateral zones, a more active organoperoxide (such as for example n-butyl-4,4-di(tert-butyl-peroxy) valerate) is used for the aforesaid central zone.
Therefore, by using organoperoxides of differing activities at about the same temperature, the in situ formation of the built-in sealant of the lateral zones is to have greater storage modulus (G′) than the storage modulus (G′) of the central zone, and, accordingly, a reduced potential tendency for a small degree of flow under conditions of centrifugal force occasioned by operation of the tire.
A further embodiment of the invention is a treatment of the precipitated silica with, for example, at least one of polyalkylene glycol (e.g. polyethylene glycol) and alkoxysilane in order to inhibit, retard and/or significantly prevent significant contact of hydroxyl groups contained on the precipitated (synthetic amorphous) silica aggregates with the organoperoxide, as well as possibly water moieties thereon.
Accordingly, in one embodiment, the precipitated silica may be treated in situ within the rubber composition prior to addition of the organoperoxide, or may be pre-treated prior to its addition to the rubber composition, with a low molecular weight polyalkylene oxide polymer, which might sometimes be referred to as a polyalkylene glycol (e.g. polyethylene glycol) and/or with an alkoxysilane.
Indeed, it is considered herein that significant challenges are presented using the precipitated silica (optionally also including the clay when used in combination with the precipitated silica), particularly when used in place of rubber reinforcing carbon black for reinforcing filler for a non-black colored sealant for the above reasons.
Therefore, as indicated above, when the precipitated silica is used, it is preferably treated with at least one of a polyalkylene oxide (e.g. polyethylene oxide) and alkoxysilane.
In a further embodiment of the invention, while the butyl rubber, as a copolymer of isobutylene and isoprene, may be composed of greater than one weight percent units derived from isoprene, it is preferred that it is composed of from only about 0.5 to 1.0 weight percent units derived from isoprene. The use of a butyl rubber with such low unsaturation content is to promote a more efficient depolymerization by treatment with the organoperoxide where it is envisioned that the presence of the double bonds within the butyl rubber may tend to terminate its depolymerization when the depolymerization process reaches the double bond unsaturation in the butyl rubber.
In an additional aspect of the invention, to promote better processing of the butyl rubber-based sealant precursor composition, it is desired to use a butyl rubber that has a relatively high Mooney viscosity (ML+8) value at 125° C. in a range of from about 25 to about 60, alternately from about 40 to about 60.
Thus a butyl rubber of very low isoprene-based unsaturation content (for more effective depolymerization of the butyl rubber) and relatively high Mooney viscosity (to promote better physical handling of the sealant precursor composition) is desired.
In practice, it is desired herein for the butyl rubber-based sealant precursor composition to have a storage modulus (G′) physical property, at a 5 percent dynamic strain at 100° C. and 1 hertz in a range of about 170 to about 350 kPa, alternately in a range of from about 175 to about 300 kPa.
In the description of this invention, the term “phr” is used to designate parts by weight of an ingredient per 100 parts of elastomer unless otherwise indicated. The terms “elastomer” and “rubber” are used interchangeably unless otherwise indicated. The terms “cure” and “vulcanize” are used interchangeably unless otherwise indicated.